1. Field of the Invention
The present invention relates to a Zener zapping device forming a voltage setting circuit for generating a highly accurate voltage supplied to analog integrated circuitry etc., and to a Zener zapping method using the Zener zapping device.
2. Description of the Background Art
Conventionally, the Zener zapping technique has been widely used as a method for controlling variations in analog integrated circuits etc. caused in manufacture after the manufacture so as to generate highly accurate voltage. FIG. 5 is a circuit diagram showing part of a structure of a semiconductor integrated circuit. The semiconductor integrated circuit shown in FIG. 5 has a terminal 101 at which the voltage is to be set (the potential at the terminal 101 is taken as V.sub.ref), a Zener diode 106a having its one end connected to the terminal 101, a resistor 105a (having a resistance value R1) having its one end connected to the terminal 101, a Zener diode 106b having its one end connected to the other end of the resistor 105a and to the other end of the Zener diode 106a, a resistor 105b (having a resistance value R2) having its one end connected to the other end of the resistor 105a and to the other end of the Zener diode 106a, a Zener diode 106c having its one end connected to the other end of the resistor 105b and to the other end of the Zener diode 106b and its other end grounded, and a resistor 105c (having a resistance value R3) having its one end connected to the other end of the resistor 105b and to the other end of the Zener diode 106b and its other end grounded.
The semiconductor integrated circuit shown in FIG. 5 also has a resistor 104a (having a resistance value R4) having its one end connected to a voltage source 103 (having a potential VB) and its other end connected to the terminal 101, and a resistor 104b (having a resistance value R5) having its one end connected to the terminal 101 and its other end grounded. Further, the semiconductor integrated circuit shown in FIG. 5 has a terminal 108a connected to the one end of the Zener diode 106a, a terminal 108b connected to the other end of the Zener diode 106a and to the one end of the Zener diode 106b, a terminal 108c connected to the other end of the Zener diode 106b and to the one end of the Zener diode 106c, and a terminal 108d connected to the other end of the Zener diode 106c.
Generally, when a Zener voltage in reverse direction is not applied to a Zener diode, the Zener diode is in an open state between its one end and the other end. When an excessive current in the reverse direction is instantaneously passed to the Zener diode, the Zener diode causes a Zener breakdown and one end and the other end of the Zener diode are short-circuited.
FIG. 6 is a circuit diagram showing an example of a voltage setting circuit for setting the potential V.sub.ref. In FIG. 6, the part surrounded by the one-dot chain line corresponds to the semiconductor integrated circuit shown in FIG. 5, and the outside of the one-dot chain line is a Zener zapping device connected to the semiconductor integrated circuit. A current source 102 has its one end grounded, and the grounded end is connected to the terminal 108c and its other end is connected to the terminal 108a, so that a current I is supplied from the current source 102 to the terminal 108a. Then a current I1 flows to the Zener diodes 106a and 106b in the reverse direction to cause the Zener diodes 106a and 106b to undergo Zener breakdown. While part of the current I flows also to the resistors 104b, 105a, and 105b as a current I2, it is possible to cause Zener breakdown at the Zener diodes 106a and 106b by setting the current value of the current I sufficiently large.
With the Zener breakdown of the Zener diodes 106a and 106b, one end and the other end of the Zener diode 106a and one end and the other end of the Zener diode 106b are respectively short-circuited. As a result, one end and the other end of the resistor 105a connected in parallel to the Zener diode 106a and one end and the other end of the resistor 105b connected in parallel to the Zener diode 106b are shorted respectively by the Zener diodes 106a and 106b, and then the resistors 105a and 105b do not function as resistance from the circuit standpoint. In this case, the potential V.sub.ref at the terminal 101 is given as (R5//R3).multidot.VB/(R4+(R5//R3)).
As stated above, the combined resistance value of the resistors 104a, 104b, 105a to 105c can be varied by causing arbitrary ones of the Zener diodes 106a to 106c to undergo Zener breakdown to short both ends of arbitrary ones of the resistors 105a to 105c, which enables the potential V.sub.ref at the terminal 101 to be highly accurately set to a desired value.
However, such a conventional Zener zapping device has the following problems. FIG. 7 is a circuit diagram showing another example of the voltage setting circuit, which is intended particularly to cause the Zener diodes 106a and 106c to undergo Zener breakdown. A current source 102a has its one end grounded, and the grounded end is connected to the terminal 108b and its other end is connected to the terminal 108a; a current source 102b has its one end grounded, and the grounded end is connected to the terminal 108d and its other end is connected to the terminal 108c.
Passing a reverse current from the current source 102a to the Zener diode 106a through the terminal 108a causes the Zener diode 106a to undergo a Zener breakdown, and passing a reverse current from the current source 102b to the Zener diode 106c through the terminal 108c causes the Zener diode 106c to undergo a Zener breakdown.
However, when the current Ib is supplied from the current source 102b to the terminal 108c, part of the current Ib, the current Ib2, flows to the terminal 108b through the Zener diode 106b. Accordingly, when the current Ia from the current source 102a and the current Ib from the current source 102b are supplied at the same time, the current Ib2 functions as a current in the forward direction for the Zener diode 106a to clamp the potential at the terminal 108b, so that the Zener diode 106a cannot cause a Zener breakdown. Accordingly, when causing the Zener diodes 106a and 106c to undergo Zener breakdown in the voltage setting circuit shown in FIG. 7, it is necessary to separately supply the current Ia from the current source 102a and the current Ib from the current source 102b, which causes the problem that the Zener-zapping takes long time. Further, the need of the two current sources 102a and 102b causes the device scale of the Zener zapping device to be large.